Whether on the ground or in the air, a variety of forces contribute to the motion of a ball. Focusing on spherical balls -- such as those used in golf, baseball and basketball -- their size and mass help determine how they travel. In addition to having an impact on speed and trajectory, the size and mass of balls also contribute to how external forces affect them.Want to get in great shape? Learn more about LIVESTRONG.COM's nutrition and fitness program!
Physics refers to balls such as those used in competitive sports as "rigid bodies." For rigid bodies in motion, the principles of the physics field called "dynamic" apply. Among the factors that help to determine how a rigid body moves, its mass per unit length, or mass-density, plays a vital role both on the ground and in the air. This helps determine how the laws of inertia apply. An object with high mass-density proves more difficult to move and also more difficult to stop once it begins moving.
Objects in Flight
The difference between softballs and the shots used in shot putting provide a great example of the effects of mass-density. Although both are approximately equal in diameter, you'll find the mass of the heavy metal shot much higher than that of the softball. As such, shot putting requires a greater amount of strength, or initial force, than throwing a softball. Due to the shot's high mass-density, however, forces such as air friction and the downward pull of gravity affect its flight less than a softball's.
Objects on the Ground
While the law of inertia works in a similar fashion on both the air and the ground, the forces that help to determine how a ball rolls differ slightly between the two. On level ground, gravity plays almost no role in slowing an object's movement, with air friction also factoring little into how an object rolls. Although rotational force, or spin, remains important in the air, it plays a significantly greater role in determining how a ball moves on a level surface. In addition to the rotational momentum used to roll a ball forward, angled spin helps a ball to curve, having a greater effect on balls with lower mass-density. In place of the forces of gravity and air friction, surface friction plays a major role in determining how balls roll. Similar to balls in flight, surface friction impedes the movement of balls with low mass-density, such as soccer balls, more than it does those with high mass-density, such as bowling balls.
Initial Force and Motion
Due to the different effects of external forces, balls with high mass-density would travel farther and faster than those with low mass-density if you could launch both with an infinite amount of force. Although you can't do this, throwing or rolling balls of different mass-densities at the same initial speed obtains the same effect. Hitting a golf ball with more power than a ping pong ball provides illustration, with the golf ball sent soaring and air friction ending the ping pong ball's flight shortly after it leaves the tee.
You can see similar effects on the ground. While it would require a greater amount of force to roll both at the same initial speed, a bowling ball's higher mass-density would give it greater rotational force than a rolled basketball. Surface and air friction would soon slow the basketball down, eventually stopping it while its high mass-density and rotational force continued to carry the bowling ball.